1. Field of the Invention
The invention refers to a fuel injection nozzle, in particular a pump nozzle comprising a nozzle needle being loaded by a spring in a closing sense, in which the pressure space located upstream the seat of the nozzle needle is in open connection with the storage space of a yield piston being shiftable within a guide bore and being loaded by a spring in a direction towards the storage space. The term "pump nozzle" is used in this case to denote an injection means in which the injection nozzle is connected with the pump piston bushing and the pump piston to form one single constructional unit.
2. Description of the Prior Art
A device of the initially mentioned type may, for example, be derived from DE-OS 34 09 924. The arrangement of the yield piston, which is connected in parallel to the nozzle needle with respect to the action of the pressurized fluid, serves, in this case, the purpose to subdivide the fuel injection step into a preliminary injection step and a separate main injection step. For this purpose, the nozzle needle is first lifted against the force of the nozzle needle spring on occasion of a pressure rise within the pressure conduit for the fuel, whereby the injection step is started. Thereafter, also the yield piston is shifted against the force of the nozzle needle spring on account of the pressure rise within the pressure conduit, whereby, on the one hand, the spring force acting on the nozzle needle spring is increased and, on the other hand, the pressure is reduced for a short time interval on account of the activated yield volume of the pressure reduction results in the nozzle needle assuming a closed position for a short time. The pressure becoming increased thereafter is then in the position to lift against the nozzle needle against the pressure of the now stronger prestressed nozzle needle spring, so that the main injection step is started. The amount of fuel injected during the preliminary injection step is now strongly dependent on the rotational speed. With the engine rotating with the desired low idle speed, the yield piston has at disposal a longer time interval for effecting its yielding movement, so that the preliminary injected fuel amount is too strongly reduced, while the preliminary injected fuel amount is increased at higher rotating speeds, relative to the preliminary injected fuel amount as is injected in case of idle speed. This is not desired, because this results in noise generation at a low speed and in an incomplete combustion at high speeds. Also, the time interval between the end of the preliminary injection step and the beginning of the main injection step is not well adapted to the duration of the ignition delay. This time interval shall become shorter with increasing rotational speed and shall vanish at a definite rotational speed. The same applies also in case of increasing load. This is to mean that this time interval shall become zero within a definite area in the motor performance graph. This is not the case in the known arrangement, which gives rise to an excessive long duration of the total injection step, which results in an incomplete combustion. Furthermore, the yield piston, nozzle needle spring and nozzle needle form an oscillatory system, which is subject to become vibratory within a broad range of rotational speeds of a Diesel engine. Oscillations of the yield piston and of the nozzle needle result, irrespective of the increased mechanical stress, in a reduced fuel supply during the main injection step, which results in a longer duration of the main injection step. From AT-PS 292 382, it has become known to provide a throttling means within the conduit leading to the storage space of the yield piston. However, this is disadvantageous because the opening pressure is influenced by this throttling means and because formation of gas bubbles is caused, which changes the compressibility of the fluid within the storage space. Thus, it becomes difficult to control the volumetric amount of fuel flowing into the storage space of the yield piston.